1. Field of the Invention
The present invention relates to a machine tool, in particular a long-bed machine tool, with a column that can be displaced in at least a first axis, and with at least one spindle, clamped rotatably at support points in the region of its two outer ends, that extends parallel to the first axis and is joined to the column in such a way that when the spindle rotates, the column is displaced in the first axis.
2. Description of the Related Art
It is repeatedly observed with machine tools of this kind that at very high traverse speeds or with long-bed machine tools in which the clamped length of the spindle is very great, the spindle tends to vibrate undesirably.
These vibrations not only put an upper limit on the manageable traverse speed and bed length, but also lead to premature material fatigue and possibly to failure of the machine tool in question. Furthermore, these vibrations impair the processing accuracy of the machine tool, since under such operating speeds conditions the column can no longer be displaced accurately at high speed over longer distances.
It is known that the critical rotation speed at which shafts begin to vibrate depends on the protruding length and the diameter of the shaft. Although the critical speed can be increased by reducing the clamped free length or by increasing the diameter, this leads either to an undesirably high spindle weight or to insufficient bed length. When the spindle diameter is increased, in particular, another consideration is that the deflection due to its own weight is in itself sufficient to provoke the undesirable spindle vibration. Moreover, these vibrations can build up to the point that considerable damage can be done to the machine tool.
On this basis, the object of the present invention is to develop a machine tool of the aforesaid type in such a way that high traverse speeds can be achieved and/or long bed lengths are possible with long-bed machine tools. The new machine tool is intended to be of very simple design.
German patent DE-C-31 39 031 discloses a machine tool with a displaceable workpiece table and a driving spindle therefore which is provided with a damping apparatus acting on the spindle between support points thereof to damp vibrations of the driving spindle. The damping apparatus comprises two auxiliary bearings in the longitudinal direction of the spindle slidingly supported by the workpiece table. The auxiliary bearings are each provided at opposite ends of the workpiece table between same and a respective support point of the spindle.
This construction is expensive and complicated and has the further disadvantages that the known damping apparatus can not be retrofitted to existing machine tools and narrows the travelling distance of the workpiece table undesirably.
A further, also very complicated damping apparatus for a driving spindle of a machine tool is known from German utility model DE 90 17 035. This apparatus comprises a fixed support groove encircling the spindle from below to about 70% of the circumference thereof and extending along almost all of the length of the driving spindle.
This damping mechanism also has the disadvantages mentioned above, i.e. it can not be retrofitted to existing machine tools.
In view of this it is an object of the present invention to provide a machine tool of the kind mentioned at the outset which is of very simple design, whereby it is intended that the damping apparatus may be retrofitted to existing machine tools.